Torque transmitting devices such as clutches or brakes are used throughout the automotive industry. For example, vehicle transmissions employ multiple clutches to engage and disengage the gearsets of the transmission to provide forward and reverse gear ratios.
Generally, a clutch includes a friction plate and a reaction plate. The friction plate has a layer of friction material attached to the surface that opposes the reaction plate. Conventional friction materials are (a) cellulose/Kevlar/resin based materials; (b) sintered metallic fiber friction materials; and (c) woven carbon fiber friction materials.
Cellulose/Kevlar/resin based materials are cost effective, provide high torque capacity, are porous, offer elastic structure capable of excellent load distribution without losing permeability as long as the local load is below the elasticity limits. However, in a wet clutch application, local overloading leads to restricted fluid supply through the porous structure to the surface as result of plastic deformation, closing pores, permeability loss; and since the friction material acts more as thermal insulator, such conditions lead to local overheating, shudder, glazing, structural friction material damage, hot spotting and judder. Heavily relying on friction modifier additives to avoid stick/slip (shudder) behavior may only temporarily produce the desired effects. For example, as the friction modifier additive concentration drops below a critical level due to depletion, the shudder amplitude increases.
Sintered metallic friction materials have much better heat transfer capability and heat resistance and are capable of taking high overload without structural damage. Moreover, an optimized load distribution is achieved by abrasive wear. However, sintered metallic friction materials are less cost effective, since the applied load distribution is slightly different for each engagement. Abrasive wear is much less effective as compared to elastic deformation to avoid the local overloading. Since fluid is squeezed between two solid bodies in a wet clutch application, much higher local fluid pressure (hard-EHL vs. soft-EHL) leads to much higher heat generation as result of fluid shear, so even better heat conductivity may not be enough to support the film, a higher kinematic viscosity fluid may be required (with an additional loss of efficiency) and resulting in a lower torque capacity.
Woven carbon fiber friction materials have benefits and drawbacks that are somewhere in the middle between cellulose/Kevlar/resin based and sintered friction materials.
Thus, there is a need for a new and improved torque transmitting device that takes advantage of the benefits of the above referenced friction materials and limits the disadvantages.